Optical-fiber preform and method for manufacturing optical-fiber preform

ABSTRACT

An optical fiber preform which can be drawn into a low attenuation optical fiber is provided with a core portion and a cladding portion surrounding the core portion. The core portion includes a first core portion and a second core portion surrounding the first core portion. The cladding portion includes a first cladding portion surrounding the second core portion and a second cladding portion surrounding the first cladding portion. The first core portion contains an alkali metal element, the concentration of oxygen molecules contained in glass is 30 mol ppb or more and 200 mol ppb or less in a part of or entire region having an alkali metal atom concentration of 100 atomic ppm or more, and the concentration of oxygen molecules contained in glass is 10 mol ppb or less in a region having an alkali metal atom concentration of 50 atomic ppm or less.

TECHNICAL FIELD

The present invention relates to an optical fiber preform and a methodfor manufacturing an optical fiber preform.

BACKGROUND ART

JP 2005-537210A (PTL 1), US 2006/0130530A (PTL 2), JP 2007-504080A,JP2008-536190A, JP 2010-501894A, JP 2009-541796A, JP 2010-526749A, WO98/002389, and U.S. Pat. No. 5,146,534B describe silica base opticalfibers in which cores are doped with alkali metal elements. It is saidthat in the case where a core portion of an optical fiber perform isdoped with an alkali metal element, the viscosity of the core portioncan be reduced during drawing of the optical fiber preform, relaxationof a network structure of a silica glass proceeds and, thereby, theattenuation of the optical fiber can be decreased.

PTL 1 and PTL 2 describe a diffusion method as a method for doping thesilica glass with the alkali metal element. The diffusion method dopesthe inside surface of the silica glass pipe with the alkali metalelement through diffusion by heating a pipe with an external heat sourceor generating plasma in the pipe while a source material vapor obtainedby heating an alkali metal or an alkali metal salt, which serves as asource material, is introduced into the silica glass pipe.

After the vicinity of the inside surface of the silica glass pipe isdoped with the alkali metal element, as described above, the diameter ofthe resulting silica glass pipe is reduced by heating. After thereduction in diameter, some thickness of the inside surface of thesilica glass pipe is etched for the purpose of removing transition metalelements, e.g., Ni and Fe, which are added at the same time withaddition of the alkali metal element. The alkali metal element diffusesfaster than the transition metal element. Therefore, even when somethickness of glass surface is etched to remove the transition metalelement, it is possible to allow the alkali metal element to remain.

After the etching, the glass pipe is heated and collapsed, so that analkali metal element-doped rod (core portion of an optical fiberpreform) is produced. A glass serving as a cladding portion having arefractive index smaller than the refractive index of the core portionis applied around the perimeter of the core portion, so that the opticalfiber preform is produced. Then, an optical fiber can be produced bydrawing the resulting optical fiber preform by a known method.

SUMMARY OF INVENTION Technical Problem

It is an object of the present invention to provide an optical fiberpreform which can be drawn into a low attenuation optical fiber and amethod which can produce such an optical fiber preform.

Solution to Problem

An optical fiber preform is provided. The optical fiber preform includesa core portion and a cladding portion, the core portion contains analkali metal element, the concentration of oxygen molecules contained inglass is 30 mol ppb or more and 200 mol ppb or less in a part of orentire region having an alkali metal atom concentration of 100 atomicppm or more, and the concentration of oxygen molecules contained inglass is 10 mol ppb or less in a region having an alkali metal atomconcentration of 50 atomic ppm or less.

In the optical fiber perform according to the present invention,preferably, the maximum value of the alkali metal atom concentration is500 atomic ppm or more and the concentration of oxygen moleculescontained in glass is 30 mol ppb or more and 200 mol ppb or less in thea part of or entire region an alkali metal atom concentration of 500atomic ppm or more. Preferably, the core portion is formed form Ge-freeglass and the relative refractive index difference with reference topure silica glass is within the range of −0.1% or more and +1.0% orless.

In the optical fiber preform according to the present invention, thealkali metal atom concentration is preferably 4,000 atomic ppm or less.The Cl concentration in the core portion is preferably 500 atomic ppm orless. Preferably, the core portion includes a first core portion dopedwith an alkali metal element and a second core portion which is disposedaround the perimeter of the first core portion and which has a Clconcentration of 10,000 atomic ppm or more. Also, the average potassium(K) concentration in the entire core portion is preferably 5 to 100atomic ppm, and most preferably 10 to 30 atomic ppm.

As another aspect of the present invention, a method for manufacturingthe optical fiber preform according to the present invention isprovided. The method for manufacturing an optical fiber preformincluding a core portion and a cladding portion includes the steps offorming a solid core portion by heating a silica glass pipe containingan average concentration of 5 atomic ppm or more of alkali metal atom toa temperature of 1,600° C. or higher with an external heat source toinduce collapsing, while an oxygen partial pressure in the inside of thesilica glass pipe is maintained at 1 kPa or more and 80 kPa or less andapplying a cladding portion having a refractive index smaller than therefractive index of the core portion around the core portion.

Preferably, the collapsing is induced while the internal pressure of thesilica glass pipe is maintained at 10 kPa or more and 80 kPa or less.Meanwhile, it is preferable that the collapsing be induced while a gas,in which all inert gas is mixed with oxygen at a flow rate 0.25 or moretimes and 100 or less times the flow rate of oxygen, is fed into thesilica glass pipe.

Advantageous Effects of Invention

The optical fiber preform according to the present invention can bedrawn into a low attenuation optical fiber. Also, the method formanufacturing an optical fiber preform, according to the presentinvention, can produce such an optical fiber preform.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of an optical fiber preform according to anembodiment of the present invention.

FIG. 2 is a flow chart illustrating a method for manufacturing anoptical fiber preform, according to an embodiment of the presentinvention.

FIG. 3 is a conceptual diagram illustrating a collapsing step in themethod for manufacturing an optical fiber preform, shown in FIG. 2.

FIG. 4 is a graph showing the relationship between the oxygen moleculeconcentration in the central portion of the core portion of an opticalfiber preform and the attenuation of an optical fiber at a wavelength of1,550 nm.

FIG. 5 is a graph showing the relationship between the oxygen moleculeconcentration at the position 4 mm distant from the center of an opticalfiber preform and the attenuation of an optical fiber at a wavelength of1,550 nm.

FIG. 6 is a graph showing the relationship between the oxygen partialpressure during the collapsing step and the oxygen moleculeconcentration in the central portion of a core rod after collapsing.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below in detailwith reference to attached drawings. The present inventor conductedresearch on an optical fiber manufacturing method by using a diffusionmethod and found, in the process thereof, that the attenuation of anoptical fiber produced by the method was not always reduced. Then, thepresent inventor investigated the cause of hindrance to reduction inattenuation of the optical fiber and has completed the presentinvention.

FIG. 1 is a sectional view of an optical fiber preform 1 according to anembodiment of the present invention. The optical fiber preform 1 isformed from silica glass and is provided with a core portion 10 and acladding portion 20 surrounding the core portion 10. The refractiveindex of the core portion 10 is higher than the refractive index of thecladding portion 20. The core portion 10 includes a first core portion11 and a second core portion 12 surrounding the first core portion 11.The cladding portion 20 includes the first cladding portion (opticalcladding portion) 21 surrounding the second core portion 12 and a secondcladding portion (physical cladding portion) 22 surrounding the firstcladding portion 21. The first core portion 11 contains an alkali metalelement. This alkali metal element is preferably a potassium element.

FIG. 2 is a flow chart illustrating a method for manufacturing anoptical fiber preform, according to an embodiment of the presentinvention. In the method for manufacturing an optical fiber preformaccording to the present embodiment, optical fiber preform 1 is producedby sequentially performing a preparation step (Step S1), an additionstep (Step S2), a diameter reduction step (Step S3), an etching step(Step S4), a collapsing step (Step S5), a first elongation and grindingstep (Step S6), a first rod-in-collapsing step (Step S7), a secondelongation and grinding step (Step S8), a second rod-in-collapsing step(Step S9), and an OVD step (Step S10). In this regard, an example ofproduction condition will be described below.

In the preparation step (Step S1), a silica glass pipe, in which analkali metal element is to be diffused, is prepared. This silica glasspipe contains 100 atomic ppm of chlorine (Cl) and 6,000 atomic ppm offluorine, and the concentration of other dopants and impurities is 10mol ppm or less. The outside diameter of this silica glass pipe is 35 mmin diameter and the inside diameter is about 20 mm in diameter.

In the addition step (Step S2), potassium serving as an alkali metalelement is added to the inside surface of the silica glass pipe.Potassium bromide (KBr) is used as a source material. A KBr vapor isgenerated by heating KBr to a temperature of 840° C. with an externalheat source. A silica glass pipe is heated from the outside by anoxyhydrogen burner in such a way that the temperature of the outsidesurface of the silica glass pipe becomes 2,150° C. while the KBr vaporis introduced into the silica glass pipe with a carrier gas composed of1 SLM (1 liter/min on a standard state basis) of oxygen. At this time,the burner is traversed at a speed of 40 mm/min, and heating isperformed 15 turns in total, so that a potassium metal element is addedto the inside surface of the silica glass pipe by diffusion. The maximumvalue of the potassium concentration in this alkali metal-doped pipe is1,000 atomic ppm.

In the diameter reduction step (Step S3), the diameter of the silicaglass pipe doped with potassium is reduced. At this time, the silicaglass pipe is heated with an external heat source in such a way that theoutside surface of the silica glass pipe becomes at 2,250° C. while 0.5SLM of oxygen is passed in the inside of the silica glass pipe. Theexternal heat source is traversed and heating is performed 6 turns intotal, so that the diameter of the glass pipe doped with potassium isreduced until the inside diameter reaches 5 mm.

In the etching step (Step S4), the inside surface of the silica glasspipe is etched. At this time, vapor phase etching is performed byheating the silica glass pipe with an external heat source while a mixedgas of SF₆ (0.2 SLM) and chlorine (0.5 SLM) is introduced into theinside of the silica glass pipe. Consequently, the pipe inside surfacecontaining a high concentration of impurities added together with thealkali metal element can be removed and these impurities can be removed.

In the collapsing step (Step S5), the silica glass pipe is collapsed.FIG. 3 is a conceptual diagram illustrating the collapsing step (Step 5)in the method for manufacturing an optical fiber preform according tothe present embodiment. In the collapsing step, the oxygen partialpressure is specified to be 8 kPa by decreasing the absolute pressure inthe silica glass pipe to 97 kPa or less while a mixed gas of oxygen (0.1SLM) and He (1 SLM) is introduced into the inside of the silica glasspipe 30. In this state, the silica glass pipe 30 is collapsed byperforming heating with an external heat source 40 in such a way thatthe surface temperature of the silica glass pipe becomes 2,150° C.

A first rod (outside diameter 25 mm) containing the alkali metal elementis obtained by this collapsing. The value of the potassium atomconcentration in the first rod is 600 atomic ppm at a maximum and is 550atomic ppm on a center axis. The diameter of the region doped with 50atomic ppm or more of potassium is 8 mm. The dissolved oxygen moleculeconcentration in the first rod can be measured on the basis of theintensity of fluorescence at a wavelength of 1,272 nm when the lightwith a wavelength of 765 nm is applied (refer to, for example, K.Kajihara, et al., J. Ceramic Soc. Japan 112 [10], pp. 559-562 (2004)).The dissolved oxygen molecule concentration is at a maximum in thevicinity of the central axis portion of the first rod and the valuethereof is, for example, 52 mol ppb.

In the elongation and grinding step (Step S6), the first rod obtained bycollapsing is elongated in such a way that the diameter becomes 20 mmand, thereafter, the perimeter portion is ground in such a way that thediameter becomes 13 mm to produce a first core portion 11.

In the rod-in-collapsing step (Step S7), a second core portion 12 isdisposed around the first core portion 11 and, thereby, a second rod isobtained. At this time, the second rod is formed by the rod-in-collapsemethod, where the first core portion 11 is inserted into the inside ofthe silica glass pipe (second core portion 12) which is doped with 5,000atomic ppm of Cl atom and which has an outside diameter of 65 mm andboth of the first and second core portions are heated and integratedwith an external heat source.

In the second elongation and grinding step (Step S8), the second rod iselongated in such a way that the diameter becomes 24 mm and, thereafter,the perimeter portion is ground in such a way that the diameter becomes20 mm. As a result, the ratio D2/D1 of the diameter D2 of the secondcore portion to the diameter D1 of the first core portion becomes 3. Thefirst core portion 11 and the second core portion 12 are merged into thecore portion 10.

In the second rod-in-collapsing step (Step S9), a first cladding portion21 is disposed around the core portion 10. At this time, arod-in-collapse method is used, where the core portion 10 is insertedinto the inside of the silica glass pipe (first cladding portion 21)doped with fluorine and both of the core portion 10 and first claddingportion 21 are heated and integrated with an external heat source. Themaximum relative refractive index difference between the second coreportion 12 and the first cladding portion 21 is about 0.34%. As a resultof synthesis by this rod-in-collapse method, the amount of water in thecore portion 10 and the first cladding portion 21 in the vicinitythereof can be decreased to a sufficiently low level.

In the OVD step (Step S10), the rod produced by integrating the coreportion 10 and the first cladding portion 21 is elongated to apredetermined diameter, and thereafter, a second cladding portion 22containing fluorine is synthesized by the OVD method around the rod toproduce an optical fiber preform 1. In the resulting optical fiberpreform 1, the outside diameter of the first cladding portion 21 is 36mm and the outside diameter of the second cladding portion 22 is 140 mm.The maximum relative refractive index difference between the second coreportion 12 and the second cladding portion 22 is about 0.32% at amaximum. Meanwhile, the OH group concentration outside the firstcladding portion 21 can be measured by using infrared absorptionspectroscopy and is about 400 mol ppm.

In the following drawing step, an optical fiber can be obtained bydrawing the optical fiber preform 1 produced by the above-describedmethod for manufacturing an optical fiber preform. The drawing speed is2,300 m/min, and the drawing tension is 0.5 N.

The optical fiber preform 1 was produced under conditions describedabove and the optical fiber was further produced. Characteristics of theresulting optical fiber are as shown in Table below and, therefore, theoptical fiber exhibiting low attenuation was obtained.

TABLE Charac- teristic Item Unit value Potassium average value atomicabout 3 concentration in core ppm Attenuation @ 1300 nm dB/km 0.287 @1380 nm dB/km 0.292 @ 1550 nm dB/km 0.162 Chromatic dispersion @ 1550 nmps/nm/km +15.9 Dispersion slope @ 1550 nm ps/nm²/km +0.054 Zerodispersion nm 1310 wavelength Dispersion slope @ zero dispersionps/nm²/km +0.083 wavelength Effective area @ 1550 nm μm² 82 Mode fielddiameter @ 1550 nm μm 10.3 @ 1310 nm μm 9.1 Fiber cutoff on 2 m lengthof nm 1310 wavelength fiber Cable cutoff on 22 m length of nm 1230wavelength fiber Polarization mode C band, L band ps/√km 0.11 dispersionNonlinear coefficient @ 1550 nm, random (W · km)⁻¹ 1.1 polarizationstate

In an modified example, a core portion containing an alkali metalelement was produced under the same condition except that in theaddition step (Step S2), the temperature of KBr was specified to be 800°C. and, therefore, the addition step was performed while theconcentration of the KBr vapor was reduced. The value of the potassiumatom concentration in this core portion was 200 atomic ppm at a maximumand was 150 atomic ppm on a center axis. The diameter of the regiondoped with 10 atomic ppm or more of potassium was 6 mm. The dissolvedoxygen molecule concentration in the core portion was at a maximum inthe vicinity of the central axis portion of the first rod and the valuethereof was, for example, 180 mol ppb. An optical fiber was produced bydrawing the resulting optical fiber preform. As a result, theattenuation of the optical fiber at a wavelength of 1,550 nm was 0.165dB/km.

In a comparative example, an optical fiber preform was produced, wherethe condition of the collapsing step (Step S5) was changed. In thecollapsing step, the absolute pressure in the silica glass pipe wasdecreased to 97 kPa or less and 80 kPa or more while oxygen (1 SLM) wasintroduced into the inside of the silica glass pipe, and the silicaglass pipe was collapsed while the surface temperature was specified tobe 2,150° C. with an external heat source. A core portion (outsidediameter 25 mm) containing an alkali metal element was produced by thiscollapsing. The value of the potassium atom concentration in this coreportion was 500 atomic ppm at a maximum. The diameter of the regiondoped with 10 atomic ppm or more of potassium was 6 mm. The dissolvedoxygen molecule concentration in the core portion was at a maximum inthe vicinity of the center axis of the first rod and the value thereofwas 234 mol ppb. An optical fiber was produced by drawing the opticalfiber preform according to the comparative example. As a result, theattenuation of the optical fiber at a wavelength of 1,550 nm was 0.185dB/km.

As is clear from the description above, the magnitude of attenuation ofthe optical fiber is changed depending on the condition (and thecomposition distribution of the optical fiber preform based on thiscondition) of the collapsing step (Step S5) in the method formanufacturing an optical fiber preform. The present inventor conductedintensive research on the relationship between the condition of thecollapsing step, the composition distribution of the optical fiberpreform, and the attenuation of the optical fiber and determined thecondition of the collapsing step and the composition distribution of theoptical fiber preform to obtain a low attenuation optical fiber.

In the first core portion 11 of the optical fiber preform 1 according tothe present embodiment, the concentration of oxygen molecules containedin glass is 30 mol ppb or more and 200 mol ppb or less in a part of orentire region having an alkali metal atom concentration of 100 atomicppm or more, and the concentration of oxygen molecules contained inglass is 10 mol ppb or less in a region having an alkali metal atomconcentration of 50 atomic ppm or less. In this regard, preferably, thecore portion 10 is formed from Ge-free glass and the relative refractiveindex difference with reference to pure silica glass is within the rangeof −0.1% or more and +1.0% or less. Consequently, a scattering loss dueto Ge and the like can be reduced, so that the attenuation at awavelength of 1,550 nm can be reduced to 0.158 dB/km.

In the optical fiber preform 1 according to the present embodiment, thealkali metal atom concentration is preferably 4,000 atomic ppm or less.In the case where the alkali metal atom concentration is over 4,000atomic ppm, the alkali metal atom consumes the oxygen atom in the glassand, thereby, the glass is brought into an oxygen-deficient state, sothat an oxygen deficient type glass defect is generated. The attenuationis increased by the absorption loss derived from this glass defect, sothat low attenuation cannot be achieved. Therefore, the alkali metalatom concentration is preferably 4,000 atomic ppm or less.

In the optical fiber preform 1 according to the present embodiment, theCl concentration in the core portion 10 is preferably 500 atomic ppm orless. In the case where the Cl concentration is over 500 atomic ppm,oxidizing components, e.g., oxygen and chlorine, increase in the glassand an oxygen excess type glass defect is generated easily. Theattenuation is increased by the absorption loss derived from this glassdefect, so that low attenuation cannot be achieved. Therefore, the Clconcentration in the core portion 10 is preferably 500 atomic ppm orless.

In the optical fiber preform 1 according to the present embodiment,preferably, the core portion 10 includes the first core portion 11 dopedwith the alkali metal element and the second core portion 12 which isdisposed around the perimeter of the first core portion 11 and which hasa Cl concentration of 10,000 atomic ppm or more. The alkali metalelement has a high diffusion rate and, therefore, diffuses to theoutside of the fiber core during the drawing step. At this time, theperipheral portion not doped with oxygen is brought into anoxygen-deficient state easily because of diffusion of the alkali metalelement, and it is estimated that an increase in the loss occursthereby. Consequently, it is preferable that generation of oxygendeficient type defect due to diffusion of the alkali metal element besuppressed by adding a high concentration of Cl to the glass, into whichthe alkali metal element diffuses.

In the optical fiber preform 1 according to the present embodiment, theaverage K concentration in the entire core portion 10 is preferably 5 to100 atomic ppm, and most preferably 10 to 30 atomic ppm. The alkalimetal element in the core portion 10 is bonded to chlorine to generatean alkali chloride. It is estimated that the resulting alkali chloridecoagulates in the glass and causes a scattering loss. In the case wherea core in which the average concentration of alkali metal element in theentire core portion 10 was more than 30 atomic ppm was producedactually, an increase in the loss occurred and this increase in the losswas considerable in the case where 100 atomic ppm was exceeded.Therefore, the average K concentration in the entire core portion 10 ispreferably 5 to 100 atomic ppm, and most preferably 10 to 30 atomic ppm.

The method for manufacturing an optical fiber preform, according to thepresent invention, is a method for manufacturing the optical fiberpreform according to the present invention, where the solid core portion10 including the first core portion 11 is formed by heating a silicaglass pipe containing an average concentration of 5 atomic ppm or moreof alkali metal atom to a temperature of 1,600° C. or higher with anexternal heat source to induce collapsing, while the oxygen partialpressure in the inside of the silica glass pipe is maintained at 1 kPaor more and 80 kPa or less, and applying the cladding portion 20 aroundthe core portion 10 to produce the optical fiber preform 1 including thecore portion 10 and the cladding portion 20. It is preferable that thecollapsing be induced while the internal pressure of the silica glasspipe is maintained at 10 kPa or more and 80 kPa or less. Also, it ispreferable that the collapsing be induced while a gas, in which an inertgas is mixed with oxygen at a flow rate 0.25 or more times and 100 orless times the flow rate of oxygen, is fed into the silica glass pipe.

FIG. 4 is a graph showing the relationship between the maximum oxygenmolecule concentration in the central portion of the core portion of anoptical fiber preform and the attenuation of an optical fiber at awavelength of 1,550 nm. The alkali metal atom concentration in thecentral portion of the core portion of the optical fiber preform was 500atomic ppm at a maximum. If the oxygen molecule concentration is smallerthan 30 mol ppb, the attenuation increases sharply. The reason for thisis considered that the glass is brought into an oxygen-deficient stateand, thereby, an oxygen deficient type defect is generated. Meanwhile,the attenuation increases when the oxygen molecule concentration is 200mol ppb or more. The reason for this is considered that a glass defectwas generated because oxygen was excessive.

FIG. 5 is a graph showing the relationship between the oxygen moleculeconcentration at the position 4 mm distant from the center of an opticalfiber preform and the attenuation of an optical fiber at a wavelength of1,550 nm. The oxygen molecule concentration in the central portion ofthe core portion of the optical fiber preform was 200 mol ppb, and thealkali metal element concentration at the position 4 mm distant from thecenter was 50 atomic ppm. When the oxygen molecule concentration is 10mol ppb or more in the region having an alkali metal atom concentrationof 50 atomic ppm or less, the attenuation increases sharply. The reasonfor this is considered that in the region in which the alkali metalatoms are present, oxygen molecules are consumed by the reaction of theoxygen molecule with the alkali metal atom, whereas in the region inwhich the alkali metal atom concentration is low, oxygen is not consumedand becomes excessive and, thereby, an oxygen excess glass defect isgenerated easily.

FIG. 6 is a graph showing the relationship between the oxygen partialpressure during the collapsing step and the oxygen moleculeconcentration in the central portion of a core rod after collapsing. Acore rod obtained by performing the collapsing step under the conditionin which the oxygen partial pressure was 1 kPa or more and 80 kPa orless exhibited the above-described oxygen molecule concentration atwhich a low attenuation was obtained.

In the collapsing step, the oxygen partial pressure in the silica glasspipe can be specified to be a predetermined value by specifying theinternal pressure of the silica glass pipe to be 10 kPa or more and 80kPa or less. In this regard, in the case where the internal pressure ofthe silica glass pipe is specified to be less than 10 kPa, a pressuredifference between the inside and the outside of the silica glass pipeincreases, so that deformation and crush of the silica glass pipe arecaused or reduction in the yield is caused on the basis of an occurrenceof crystallization on the inside surface of the silica glass pipe due toan occurrence of crush at a low temperature.

In the collapsing step, the oxygen partial pressure in the silica glasspipe can be specified to be a predetermined value by feeding a gas, inwhich an inert gas is mixed with oxygen at a flow rate 0.25 or moretimes and 100 or less times the flow rate of oxygen, into the silicaglass pipe. At this time, N₂, Ar, He, or the like is used as the inertgas and, in particular, He is preferably used. In this regard, if a gas(in particular, active gas) other than He is used, the gas remains inthe glass after collapsing, generates bubbles and split phases, causeschanges in outside diameter and break of an optical fiber in adownstream step, and brings about occurrences of defects.

INDUSTRIAL APPLICABILITY

A low attenuation optical fiber suitable for digital coherentcommunications can be produced.

The invention claimed is:
 1. An optical fiber preform comprising a coreportion and a cladding portion, wherein the core portion contains analkali metal element, the concentration of oxygen molecules contained inglass is 30 mol ppb or more and 200 mol ppb or less in a part of orentire region having an alkali metal atom concentration of 100 atomicppm or more, and the concentration of oxygen molecules contained inglass is 10 mol ppb or less in a region having an alkali metal atomconcentration of 50 atomic ppm or less.
 2. The optical fiber preformaccording to claim 1, wherein the core portion is made from Ge-freeglass, and the relative refractive index difference with reference topure silica glass is within the range of −0.1% or more and +1.0% orless.
 3. The optical fiber preform according to claim 1, wherein thealkali metal atom concentration is 4,000 atomic ppm or less.
 4. Theoptical fiber preform according to claim 1, wherein the Cl concentrationin the core portion is 500 atomic ppm or less.
 5. The optical fiberpreform according to claim 1, wherein the core portion includes a firstcore portion doped with an alkali metal element and a second coreportion which is disposed around the perimeter of the first core portionand which has a Cl concentration of 10,000 atomic ppm or more.
 6. Theoptical fiber preform according to claim 1, wherein the average Kconcentration in the entire core portion is 5 to 100 atomic ppm.
 7. Amethod for manufacturing the optical fiber preform according to claim 1,comprising the steps of: forming a solid core portion by heating asilica glass pipe containing an average concentration of 5 atomic ppm ormore of alkali metal atom to a temperature of 1,600° C. or higher withan external heat source to induce collapsing, while an oxygen partialpressure in the inside of the silica glass pipe is maintained at 1 kPaor more and 80 kPa or less; and applying a cladding portion around thecore portion, wherein the resulting optical fiber preform includes thecore portion and the cladding portion having a refractive index smallerthan the refractive index of the core portion.
 8. The method formanufacturing the optical fiber preform, according to claim 7, whereinthe collapsing is induced while the internal pressure of the silicaglass pipe is maintained at 10 kPa or more and 80 kPa or less.
 9. Themethod for manufacturing the optical fiber preform, according to claim7, wherein the collapsing is induced while a gas, in which an inert gasis mixed with oxygen at a flow rate 0.25 or more times and 100 or lesstimes the flow rate of oxygen, is fed into the silica glass pipe.